1. Field of the Invention
The present invention generally relates to novel compounds which are useful in the treatment and prevention of disorders mediated by natriuretic peptides or proteins. More particularly, the present invention relates to novel peptides, pharmaceutical compositions comprising one or more novel peptides described herein, and their use in methods of treating or preventing ocular disorders, such as glaucoma, ocular hypertension, and optic neuropathies, cardiovascular disease, kidney disease, lung disease, and other disorders mediated by natriuretic peptides or proteins.
2. Description of Related Art
The natriuretic peptides (NP's) are a family of cyclic peptide hormones that have first been described by their involvement in the regulation of natriuresis, diuresis and blood pressure control. To date, four natriuretic peptides have been discovered in man, i.e. atrial natriuretic peptide (ANP; SEQ ID NO:1), B-type or brain natriuretic peptide (BNP; SEQ ID NO;2), C-type natriuretic peptide (CNP; SEQ ID NO:3) and urodilatin (SEQ ID NO:4) (see FIG. 1; and Cho et al., 1999, Heart Dis. 1:305-328). All NP's are synthesized as prepro-hormones which are activated by proteolytic cleavage before their release into the circulation. The NP's bind to natriuretic peptide receptors (NPR), a group of 3 different membrane bound receptors with guanylyl cyclase activity (Pandey 2005, Peptides 26:901-932).
ANP was first discovered as a blood pressure decreasing factor in rat atrial homogenates in 1981 (de Bold 1981, Life Sci 28:89-94). Human pre-pro-ANP (SEQ ID NO: 5) contains 151 amino acids and is stored after N-terminal cleavage as 126 amino acid pro-ANP (SEQ ID NO:6), predominantly in atrial granules. Cardiac stretch, due to systemic volume overload induces the rapid release of ANP from these stores. Upon secretion into the circulation, the C-terminal part of pro-ANP is cleaved by the atrial peptidase corin to the biologically active 28 amino acid form of ANP (SEQ ID NO:1) (Yan 2000, Proc Natl Acad Sci 97:8525-8529). The remaining N-terminal part can be further cleaved into 3 different hormones. i.e. Long Acting Natriuretic Peptide (LANP, amino acids 1-30; SEQ ID NO:7), Vessel Dilator (VSDL, amino acids 31-67; SEQ ID NO:8) and Kaliuretic Peptide (KP, amino acids 79-98; SEQ ID NO:9) (Vesely 2004, Eur J Clin Invest 34:674-682).
After BNP was discovered in porcine brain as a factor that showed smooth muscle relaxing activity (Sudoh T, 1988, Nature 332:78), a much greater tissue expression was found in preparations of cardiac ventricles (Mukoyama 1991, J Clin Invest 87:1402-1412), which led to the conclusion that BNP is, similarly to ANP, a cardiac peptide hormone. Although BNP can be found in storage granules in the atria, the expression in ventricles is transcriptionally regulated (Tamura 2000, Proc Natl Acad Sci 93:4239-4244). Synthesis of pre-pro-BNP is induced through cardiac wall stretch and leads to a 134 amino acid long peptide (SEQ ID NO:10) which is further cleaved by an unknown protease to yield the 108 amino acid long pro-BNP (SEQ ID NO:11). Additional cleavage liberates the active 32 amino acid C-terminal fragment of BNP (SEQ ID NO:2) and the inactive 76 amino acid N-terminal fragment also referred to as NT-pro-BNP (SEQ ID NO:12). To date, no known splice variants of human BNP exists.
CNP was first isolated from porcine brain almost 10 years after the discovery of ANP (Sudoh 1990, Biochem Biophys Res Comm 168:863-870). It is primarily expressed in the central nervous system and endothelial cells. Unlike other NP's, CNP is nearly not present in cardiac tissue, which suggest a more paracrine function on vascular tone and muscle cell growth. The 126 amino acid precursor molecule pro-CNP (SEQ ID NO: 13) is processed by the intracellular endoprotease furin into the mature 53 amino acid peptide CNP-53 (SEQ ID NO:14), which is the most abundant form in the brain (Totsune 1994, Peptides 15:37-40), endothelial cells (Stingo, 1992, Am J Phys 263:H1318-H1321) and the heart (Minamino 1991, Biochem Biophys Res Comm 179:535-542). In both, cerebral spinal fluid (Togashi 1992, Clin Chem 38:2136-2139) and human plasma (Stingo 1992, Am J Phys 263:H1318-H1321) the most common form is CNP-22 (SEQ ID NO:3), which is generated from CNP-53 by an unknown extracellular protease. Unlike the other NP's CNP-22 lacks the C-terminal extension of the 17 amino acid ring (see FIG. 1).
ANP (SEQ ID NO:1), BNP (SEQ ID NO:2) and CNP (SEQ ID NO:3) show a highly conserved amino acid sequence among different vertebrate species (see FIG. 1; and Cho 1999, Heart Dis. 1:305-328). The NP's are inactivated by two distinct mechanisms, i.e. enzymatic cleavage through neutral endopeptidases and binding to the NP clearance receptor (NPR-C; SEQ ID NO:15), which is followed by internalization and intracellular degradation of the NP (Stoupakis 2003, Heart Dis. 5:215-223).
The discovery of the natriuretic peptides ANP, BNP and CNP was followed by the description and cloning of their specific receptors, natriuretic peptide receptor-A, -B and -C (NPR-A, -B, -C) (Fuller 1988, J Biol. Chem. 263:9395-9401; Chang 1989 Nature 341:68-72; Chinkers 1989, Nature 338:78-83). NPR-A (SEQ ID NO:16) preferentially binds ANP and BNP, while NPR-B (SEQ ID NO:17) is most specific for CNP and NPR-C (SEQ ID NO:15) binds all natriuretic peptides (Koller 1991, Science 252:120-123).
The primary structure of NPR-A and NPR-B contain an extracellular ligand binding domain, transmembrane domain, intracellular kinase homology domain containing phosphorylation sites and a C-terminal guanylate cyclase domain (reviewed in Misono 2005, Peptides 26:957-68). The latter classifies NPR-A and NPR-B as particulate guanylate cyclases, also known as GC-A and GC-B (E.C.4.6.1.2). In contrast, NPR-C is lacking intracellular homology domains, but evidence is increasing for NPR-C's role not only as a scavenger receptor for natriuretic peptides, but for its' functional coupling to inhibitory G-proteins and phosphoinositide turnover (Maack 1987, Science 238:675-678; Murthy and Makhlouf 1999, J Biol Chem 274:17587-17592; Anand-Srivastava 2005, Peptides 26:1044-1059). Reflecting the grade of sequence homology in natriuretic peptides, natriuretic peptide receptors show a high degree of homology in their extracellular ligand binding domains, with the calculated similarities being 41% between NPR-A and NPR-B and 29% between NPR-A and NPR-C (van den Akker 2001, J Mol. Biol. 311:923-937).
Ligand binding to NPRs requires a dimer of glycosylated receptor subunits (Fenrick et al. 1994, Mol Cell Biochem. 137:173-182; Kuhn 2003, Circ Res. 93:700-709) and is followed by a conformational change leading to activation of the guanylate cyclase domains. Subsequently, activity of particulate guanylate cyclases is regulated through phosphorylation (reviewed in Kuhn 2003, Circ Res. 93:700-709). Phosphorylation of NPRs is maximal in the basal state, while ligand binding is followed by dephosphorylation and subsequent desensitization of the receptor.
Natriuretic receptors are expressed in many tissues throughout the organism. NPR-A, NPR-B and NPR-C are present in the cardiovascular system and the kidney, with NPR-C being the most abundant receptor subtype accounting for 80% of NPR-expression in some tissues. NPR-B is present in a particularly high level in rat pineal gland, testis and ovaries. NPR-A and NPR-B ligands both induce endothelium-independent vasorelaxation, where ANP and BNP mainly act on arterial vasculature. In contrast, CNP mainly targets the venous system, with the exception of coronary arteries, that relax in response to CNP stimulation (Marton et al. 2005, Vascul Pharmacol 43:207-212). Importantly, induction of hypotension via NPR-B activation requires 10-fold higher concentrations of ligand compared to blood pressure reduction in response to NPR-A activation (Wei et al. 1993, Am J Physiol. 264:H71-H73; Woods and Jones 1999, Am J. Physiol. 276:R1443-R1452). Relaxation of smooth muscle by activation of NPR-B has been shown in a variety of tissues, including blood vessels, seminiferous tubules and uterus. Also contraction of the ocular trabecular meshwork tissue is reduced by activation of natriuretic peptide receptors, confirming functional similarities of trabecular meshwork and smooth muscle cells (Stumpff and Wiederholt 2000, Ophthalmologica 214:33-53).
Another main target organ of natriuretic peptides is the kidney. Ligands of NPR-A induce natriuresis and diuresis by a dual mechanism (reviewed in Beltowski and Wojcicka 2002, Med Sci Monit. 8:RA39-RA52): (1) increased excretion of sodium by a reduced re-uptake of sodium ions in the distal tubulus, subsequently leading also to higher retention of water in the final urine; and (2) dilation of the affluent and concomitant contraction of the effluent glomerular capillary, increasing glomerular filtration rate, at the cost of reduction of renal perfusion (Endlich and Steinhausen 1997, Kidney Int. 52:202-207). In contrast to NPR-A-specific ligands, NPR-B-specific ligands do not induce significant natri- and diuresis, and in addition, show a peculiarity regarding glomerular flow regulation: CNP was shown to dilate both affluent and effluent capillaries in the glomerulus, thus increasing renal blood flow, but not glomerular filtration (Endlich and Steinhausen 1997, Kidney Int. 52:202-207).
In addition to effects of NP-receptor (NPR) activation on blood pressure and kidney function, powerful effects of natriuretic peptides on proliferative processes in a variety of cell types have been documented in the literature. Antiproliferative properties of NPR activation are documented for vascular smooth muscle cells, fibroblasts of different origins, mesangial cells, cancer cells and chondrocytes (reviewed in Schulz 2005, Peptides 26:1024-1034). At least for VSMC, evidence for the involvement of the transcription factor GAX in the regulation of proliferation has given an indication as to which intracellular mechanisms might be involved in growth regulation through NPR (Yamashita et al. 1997, Hypertension 29:381-387). Though tissue growth is mainly regulated by proliferative activity, some organs feature variations in cell size to influence tissue mass. This might be a physiological process, as during endochondral ossification, when chondrocytes mature by undergoing hypertrophy, or a pathological event, as in cardiac hypertrophy, which often precedes chronic heart failure. Both of the above-mentioned events of hypertrophy are regulated by NPR-B. NPR-B deficiency causes dwarfism due to abnormal endochondral ossification, characterized by size reduction of the hypertrophic zone of the epiphyseal growth plate (Bartels et al. 2004, Am J Hum Genet. 75:27-34; Tamura et al. 2004, Proc Natl Acad. Sci. 101:17300-17305).
Quite different, a partial knock out of NPR-B in rats promoted cardiac hypertrophy, i.e. hypertrophy of cardiomyocytes (Langenickel et al. 2006, Proc Natl Acad. Sci. 103:4735-4740).
Natriuretic peptides, having activity at the natriuretic receptors, were later discovered in various tissues, as well. For example, ANP was discovered in the early 1980s as an endogenous diuretic and vasorelaxant peptide, whose principle circulating form consists of 28 amino acids (SEQ ID NO:1). Subsequently, other natriuretic peptides, such as BNP (SEQ ID NO:2) and CNP (SEQ ID NO:3), were discovered. The presence of natriuretic peptides and their receptors in ocular tissues, especially those involved in the regulation of IOP, have been demonstrated. For example, in rat and rabbit eyes, ANP, BNP, and CNP, as well as NPR-A, NPR-B, and NPR-C mRNA were found in the ciliary processes, retina, and choroid (Mittag et al. 1987, Curr Eye Res. 6:1189-1196; Nathanson 1987, Invest Ophthalmol Vis Sci. 28:1357-1364; Fernandez-Durango et al. 1995, Exp Eye Res. 61:723-729). Similar results were found in bovine ciliary processes and cultured bovine ciliary epithelial cells. (Millar et al. 1997, J Ocul Pharmacol Ther. 13:1-11; Shahidullah and Wilson 1999, Br J. Pharmacol. 127:1438-1446). The presence of the peptides and their receptors in the ciliary epithelium suggests that they may play a role in the production of aqueous humor.
In addition to the ciliary processes, natriuretic peptide receptors were also found in tissues associated with the outflow of aqueous humor. ANP binding sites were localized in the longitudinal ciliary muscle of the guinea pig. (Mantyh et al. 1986, Hypertension. 8:712-721). In cultured human TM and ciliary muscle cells, CNP is the most potent and efficacious in stimulating the production of cyclic GMP, indicating the presence of functional NPR-B. Activation of this receptor reduces carbachol-induced calcium influx. (Pang et al. 1996, Invest Ophthalmol Vis Sci. 37:1724-1731). This result predicts that activation of NPR-B should cause relaxation of these tissues. Indeed, CNP significantly decreases the carbachol-induced contraction of monkey and human ciliary muscles. (Ding and Abdel-Latif, 1997, Invest Ophthalmol Vis Sci. 38:2629-2638). Change in contractility in TM and ciliary muscle may affect the outflow facility of aqueous humor.
Cyclic GMP and compounds that increase cyclic GMP in ocular tissues, such as nitric oxide donors, have been shown to lower IOP. (Nathanson 1988, Eur J Pharmacol. 147:155-156; Becker 1990, Invest Ophthalmol Vis Sci. 31:1647-1649; Nathanson 1992, J Pharmacol Exp Ther. 260:956-965; Stein and Clack 1994, Invest Ophthalmol Vis Sci. 35:2765-2768). Since natriuretic peptides potently increase cyclic GMP production, they were predicted to lower IOP, too. In the past 20 years, the natriuretic peptides have been shown to be highly effective as IOP-lowering agents. For example, various researchers have independently shown that intravitreal injection of ANP in rabbits consistently and significantly lowers IOP. This effect lasts for many hours. (Sugrue and Viader, 1986, Eur J Pharmacol. 130:349-350; Mittag et al. 1987, Curr Eye Res. 6:1189-1196; Nathanson 1987 Invest Ophthalmol Vis Sci. 28:1357-1364; Korenfeld and Becker 1989, Invest Ophthalmol Vis Sci. 30:2385-2392; Takashima et al. 1996, Invest Ophthalmol Vis Sci. 37:2671-2677). The IOP effect of ANP correlates with an increase in cyclic GMP production in the iris-ciliary body. (Korenfeld and Becker 1989, Invest Ophthalmol Vis Sci. 30:2385-2392). Intravitreal injection of BNP (Takashima et al. 1996, Invest Ophthalmol Vis Sci. 37:2671-2677) or CNP (Takashima et al. 1998, Exp Eye Res. 66:89-96) is also highly efficacious in lowering IOP. In addition to intravitreal injection, subconjunctival (Yang et al. 1997, Chin J Ophthalmol. 33:149-151) or intracameral (Sugrue and Viader 1986, Eur J. Pharmacol. 130:349-350; Fernandez-Durango et al. 1999, Eur J. Pharmacol. 364:107-113) injection of the natriuretic peptides have been shown to be ocular hypotensive as well. Systemic administration of ANP in the rabbit, (Tsukahara et al. 1988, Ophthalmologica. 197:104-109) or human (Diestelhorst and Krieglstein 1989, Int Ophthalmol. 13:99-101) also lowers IOP. Unfortunately, it has not been possible to deliver these peptides topically due to their inability to penetrate the cornea. Therefore, these potent and efficacious IOP-lowering compounds have not been developed for such use.
There is a need for novel NPR-B agonists having improved bioavailability, as compared to isolated or synthesized natriuretic peptides, that can be used in the treatment of natriuretic peptide-mediated disorders, such as ocular disorders, diabetes-related disorders, vascular disorders, cardiac and cardiovascular pathologies, inflammation and other disorders described herein. The novel NPR-B agonists, compositions and methods of the present invention meet these needs.